It had long been suggested that cholinergic neurons in the laterodorsal tegmental nucleus of the brainstem play an important role in induction and maintenance of REM sleep. This was confirmed by studies in which single neuronal activity of the cholinergic neurons was recorded in undrugged animals repeating sleep and wakefulness ; among the studies our study using rats was characterized by in vivo identification of spike activity of the cholinergic neurons with their spike configuration to be broader than other neurons. The present study is then to clarify which neurons drive the cholinergic neurons. Rats were used in all experiments.First, whether, or how, the cholinergic neurons responded to electrical stimulation of the frontal cortex, preoptic area, mammillary body of the posterior hypothalamus, thalamus or hippocampus was examined. Most frequently they were driven orthodromically from the hypothalamic sites (preoptic area and mammillary body). Antidromic responses were obtained most frequently also from the preoptic area. These results suggest that the hypothalamus and brainstem cooperate with each other in controlling sleep and wakefulness.Then, responses of the cholinergic neurons to locally applied transmitters were examined. All subgroups of the cholinergic neurons, which were classified with their behavior during sleep and wakefulness, were inhibited by noradrenaline, being different from the neurons in the cats. Histamine had an excitatory action. Hypothalamic neurohormon prolactin, which had been known to increase REM sleep when injected into the cerebral ventricle, induced long-lasting excitation of most cholinergic neurons.In summary, excitation of the cholinergic neurons which produce REM sleep may be exerted, in addition to disinhibition from monoaminergic neurons active specifically during wakefulness, by inputs from the hypothalamus with neurohormonal factors such as prolactin.